1. Field of the Invention
The present invention relates to a multivariable transmitter which executes various computation processing tasks using various physical quantity signals.
2. Description of the Related Art
The following documents are referred to as related art regarding a multivariable transmitter.    1. U.S. Pat. No. 5,495,769    2. U.S. Pat. No. 6,529,847    3. “Advanced Sensor Technology Key to New Multivariable Transmitter” Foxboro InterKama-ISA TECH Sessions (Paper Session) 1 Nov. 1999
In addition, JP-B-H8-10169 is also referred to as a related art regarding a resonant-type pressure sensor.
A typical multivariable transmitter is designed so that two or more physical quantity signals detected from a process are input to the transmitter to calculate mass flow using microprocessors, and the mass flow thus calculated is output to a two-wire transmission line as a 4-20 mA analog current signal, for example. Alternatively, the multivariable transmitter has a communication section compatible with communication standard based protocols and outputs digital information, such as the results of computation processing, to a fieldbus.
In addition, such a multivariable transmitter as described above obtains information and data, including tuning parameters, from a higher equipment not illustrated in the drawings.
FIG. 6 is a functional block diagram illustrating an example of a related art multivariable transmitter.
In the example illustrated in FIG. 6, an aperture mechanism K such as an orifice is provided on a pipe P, and a multivariable transmitter 1 detects an upstream pressure P1 and a downstream pressure P2 of a fluid F as physical quantities and outputs a flow rate signal Fout.
More specifically, the multivariable transmitter 1 is provided with a differential pressure sensor 2 for detecting the difference between the upstream pressure P1 and the downstream pressure P2 and a static pressure sensor 3 for detecting the difference between a vacuum pressure and the upstream pressure P1 defined as a reference pressure, wherein a process temperature sensor 4 for detecting the temperature of the fluid F within the pipe P is connected to the multivariable transmitter 1.
Signals detected by the differential pressure sensor 2, the static pressure sensor 3 and the process temperature sensor 4 are introduced to the multivariable transmitter 1 as physical quantities and converted by A/D converters 10, 11 and 12 to digital signals d1, d2 and d3. These digital signals are supplied to a first microprocessor 13 and a second microprocessor 14 to be subjected to computation processing.
The pressures before and after the aperture mechanism K provided in the pipe P (the upstream pressure P1 and the downstream pressure P2) and the vacuum pressure are introduced to two diaphragm sensors, i.e., the differential pressure detection diaphragm of the differential pressure sensor 2 and the static pressure detection diaphragm of the static pressure sensor 3, through the pipe P illustrated in FIG. 6. Thus one diaphragm (the differential pressure detection diaphragm) measures the differential pressure and the other diaphragm (the static pressure detection diaphragm) measures the static pressure.
In U.S. Pat. No. 5,495,769 mentioned above, pressures before and after the aperture mechanism K are introduced to two physically different diaphragms, i.e., a volumetric differential pressure diaphragm and a strain-gauge static pressure diaphragm, using independent lead pipes.
In the document “Advanced Sensor Technology Key to New Multivariable Transmitter” mentioned above, a diaphragm sensor is formed by means of silicon etching. This diaphragm sensor is illustrated as one having virtually two diaphragms by providing a vacuum chamber within a part of the sensor's diaphragm.
More specifically, the sensor is structured so that an upstream pressure P1 and a downstream pressure P2 are introduced to the diaphragm corresponding to the differential pressure sensor, and the upstream pressure P1 and the vacuum pressure of the vacuum chamber are introduced to the diaphragm corresponding to the static pressure sensor. In this example, a strain gauge is used as the detection device.
Now referring back to the example illustrated in FIG. 6, the differential pressure signal and the static pressure signal obtained from the differential pressure sensor 2 and the static pressure sensor 3 are A/D-converted and supplied to the first microprocessor 13.
The first microprocessor 13 receives an input of digital signals d1 and d2 corresponding to output signals from the differential pressure sensor 2 and the static pressure sensor 3. The first microprocessor 13 then performs computation processing to output a digital differential pressure signal d4 and a digital static pressure signal d5.
The second microprocessor 14 receives an input of the digital differential pressure signal d4 and the digital static pressure signal d5, as well as the digital signal d3 representative of the process temperature (temperature of the fluid F) from the process temperature sensor 4. The second microprocessor 14 then performs computation processing of flow rates and outputs a digital signal d6 representative of the mass flow.
Also, the process temperature sensor 4 is formed of a resistance temperature sensor (RTD, Pt100). Then, the A/D converter 12 generates the digital signal d3 corresponding to a resistance value of the resistance temperature sensor. Also, the second microprocessor 14 calculates a temperature value from the digital signal d3 which is the resistance value.
Specifically, based on an IEC computation formula, the second microprocessor 14 calculates an initial value and performs a successive approximation method three times and calculates a temperature value. Then, according to this computation method, the maximum error (temperature error) of values of temperatures from −200° C. to 850° C. results in about 0.023° C.
An output section 15 receives an input of the digital signal d6 representative of the mass flow, D/A-converts the digital signal d6, changes the digital signal to the flow rate signal Fout appropriate for the mass flow span, and transfers the flow rate signal Fout to a two-wire transmission line or a fieldbus.
A third microprocessor 16 is a block that processes communications between the multivariable transmitter 1 and a higher equipment not illustrated in the drawing according to given communication protocols. In addition, the third microprocessor 16 and the second microprocessor 14 exchange a communication data d7 with each other.
The multivariable transmitter configured in such a manner as described above has had the following problems, however.    (1) The multivariable transmitter uses three microprocessors to perform differential pressure computation processing, static pressure computation processing, flow rate computation processing and communication processing. The transmitter thus involves a relatively large number of components and therefore is costly.    (2) Since the multivariable transmitter performs the differential pressure computation processing, the static pressure computation processing, the flow rate computation processing and the communication processing with using separate microprocessors, it has been incapable of processing based on the effective use of mutually independent information.    (3) Since the multivariable transmitter performs the differential pressure computation processing, the static pressure computation processing, the flow rate computation processing and the communication processing in a sequential manner with using three microprocessors, data simultaneity and response characteristics have been unacceptably poor.    (4) Since the multivariable transmitter is structured so that the upstream pressure P1 and the downstream pressure P2 are introduced to the differential pressure sensor 2 and the upstream pressure P1 and the vacuum pressure are introduced to the static pressure sensor 3, wherein the vacuum pressure of the vacuum chamber is introduced alternatively case by case, the arrangement of lead pipes has been unacceptably complicated.    (5) Since the differential pressure sensor 2 and the static pressure sensor 3 are blocks for outputting analog signals, the output signals may drift depending on environmental conditions, such as temperature. In addition, the A/D converters 10 and 11 must be provided for the differential pressure sensor 2 and the static pressure sensor 3 independently, thus involving an increase in the number of components used and causing the transmitter to be all the more costly.
Also, there is a problem that cost becomes high since it is necessary for the second microprocessor 14 to perform numerous computations in order to calculate a temperature value.
Further, there is a problem that computation time cannot be reduced remarkably even in the case of using polynomial approximation by regression curve approximation with respect to the IEC computation formula.
Specifically, when the IEC computation formula is approximated by a regression curve of sixth order, the computation time can be reduced about half but the temperature error increases about double. Therefore, the computation time is traded off for the temperature error.
Also, it is required that the multivariable transmitter should have small size and low power consumption.